1. Field of the Invention
The present invention relates to a coupling stud assembly for coupling together a pair of flanges. More particularly, the present invention relates to a coupling stud assembly for coupling together shafts, such as propeller shafts of merchant ships, power plants and generally high-horsepower transmission shafts having multiple sections.
2. Description of the Prior Art
It is presently known to use a stud (also called a bolt) and a nut that is straight over its length, to couple together a pair of flanges that are attached to respective shafts, such as an output shaft and a drive or transmission shaft. Depending on the application, these flanges may include: a) an integral part of the shaft, wherein the flanges are machined out of the same stock material as the shaft; or b) one flange made of two halved collar pieces, a coupling sleeve and two keys for mating with the shaft, and an integral flange end for the other mating flange. The latter configuration is used in designs where openings for receiving shafts are to be minimized, such as for propeller shafts. For ease of illustration "the integral flange" concept, as shown in FIG. 1, will be discussed herein. FIG. 1 presents a prior art coupling device for connecting a high torque transmission shaft 74 to an output shaft 76 where both flanges, 75 and 77, are an integral part of the shaft 74 and 76, respectively. For clarity only one bolt and one nut is shown even though they are a plurality located circumferentially around the flange. Flanges 75 and 77 have perpendicular mating surfaces with respect to the center line of their respective shafts. This prior art coupling device includes bolt 81 for receiving nut 83 as the means of "coupling together" the two shafts.
In the coupling device of FIG. 1, a number of equidistant circumferentially spaced holes are drilled around flange 75 and 77 so that bolts 81 may pass through the two flanges to couple together the two shafts. A predetermined torque is applied evenly to all the bolts. The torque depends on the mechanical and physical characteristics of the coupling design. These design characteristics are mainly dictated by the torque to be transmitted.
It has, however, been found that it is very difficult to maintain proper tightening and many times nuts become loosened after a short period of service. Such loosening, which is primarily attributed to vibration, causes an unbalanced load distribution on the bolts and may cause the bolts to shear. In addition, the tightening torque applied to the bolt causes slight shrinkage in the bolt diameter, thereby causing a lesser contact area between the holes in the flange and the bolt itself. Consequently, relative flange slippage can occur over time, resulting in loosening of the nuts. Slippage between flanges may result in additional vibration, and may cause the bolts to shear. Such coupling arrangements are well known to persons skilled in the art and, therefore, will not be discussed further.
Generally, a good coupling device prevents slippage at the interface of the flanges and transmits driving torque smoothly between the coupled shafts with reduced levels of vibration. Therefore, an appropriate coupling between shafts is one that transmits a driving torque with no relative movement between the coupled flanges and no loosening over time.
Another way to provide a coupling between two flanges is to have hollow studs. These hollow studs are first bolted down to a given tightness, and then are hydraulically pressurized so that the studs expand radially to make a tight contact with the inner walls of the holes formed through the flanges. The problem with this concept is that loss of hydraulic pressure may loosen the nuts.
Another known method for coupling is to use tapered holes and studs. When these studs are placed under an applied torque, it becomes very difficult to achieve proper tightening due to non-linear shrinkage of the studs and the anticipated partial expansion of the tapered flange hole. Proper mating between the tapered studs and tapered holes is very difficult to achieve and often requires machining to be performed it situ.
Another known method is to carefully machine protruding wedges between the contacting surfaces of the flanges, so that the wedges will interlock so as to prevent slippage. More friction is provided by rough surfaces, instead of smooth flat surfaces, at the interface of the flanges, thereby preventing slippage. When the coupling bolts are tighten properly the wedges interlock tightly. The main problem with this alternative is that it is very difficult to achieve shaft alignment, by having ideal wedge mating surfaces at the coupling interface.